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1. Technical Field
This invention relates to an electrostatic separator for the benificiation or separation of particulate materials and, more particularly, to a high-tension electrostatic separator including a corona classifier section for classifying particulate materials according to size, and associated method.
2. Prior Art
Electrostatic separation is based upon the ability to electrically charge particulate materials having different conductive properties and then separate such particulate materials when an external electric field is applied thereto. Three main charging mechanisms applied to electrically separated particulate materials include induction, triboelectrification, and ion bombardment. Because the electrostatic force created by these mechanisms is proportional to the surface charge of the available surface area of the particulate materials and the intensity of the electric field, physical characteristics such as size, shape and specific gravity impact this process.
In general, particulate material sizes effectively separated by a high-tension electrostatic separator is coarser than approximately 100 xcexcm. In practice, uniform feed particulate material size provides better separation efficiency. Therefore, effective sizing of the particulate materials should be addressed with high-tension electrostatic separation processes to render more effective results. Screening is one method of sizing particulate materials. However, the efficiency decreases rapidly for fine particulate materials. For particulate material sizes finer than 250 xcexcm, sizing is normally performed by classification techniques. Size classification is based upon the velocity with which particulate materials fall through a medium such as air and water, for example.
In a conventional high-tension electrostatic separator, particulate materials are commonly introduced on top of a roll-type electrode. The position of a charging (corona) electrode and a static electrode, as well as the roll-rotation speed is influenced by the characteristic of particulate materials. For particulate materials having wider size distributions, the separation process requires several stages of retreatment to obtain satisfactory separation. Accordingly, from a processing point of view, it is necessary to classify such particulate materials into narrower size fractions, prior to separation, to obtain higher separation efficiency.
It is known in prior art that a high-tension electrostatic separation process has better separation efficiency with particulate materials having narrower size distributions. It has also been established that roll-type, high-tension separators are more suitable for separating finer particulate materials while plate-type, induction separators are more suitable for separating coarser particulate materials.
A significant problem with high-tension electrostatic roll-type, separators is that the fine conducting particulate materials remain on the roll outer drum surface and are misplaced with nonconducting particulate materials. This can be attributed to fine particulate materials having a higher surface charge, less inertia/centrifugal forces, as well as being susceptible to particle entrapment.
Fine particulate materials may acquire higher charges because their specific surface area is larger than the specific surface area of a coarse particulate material. Accordingly, the electrode arrangement used to separate fine particulate materials should provide a narrower corona field, less corona current, and a wider and stronger static field. In addition, higher roll-rotation speeds should be used to insure that fine conducting particulate materials leave the electrode outer drum surface as early as possible.
Alternately, coarse particulate materials have smaller specific charges. However, such coarse particulate materials have larger centrifugal forces acting thereon because their centrifugal forces are proportional to the cube of their radius. Therefore, for separating coarse particulate materials, a significant problem is that the coarse nonconducting particulate materials leave the roll-type electrode outer drum surface too early. Also, such coarse nonconducting particulate materials can be misplaced with conducting particulate materials if their surface charges are not sufficient. Consequently, the electrode arrangement used to separate coarse particulate materials should provide a wider corona field to enhance the charging thereof. In addition, the roll-rotation speed should be lower to minimize the negative effect from the centrifugal force acting on the coarse particulate materials.
Accordingly, to obtain optimal separation performance, finer and coarser fractions of particulate materials should be classified and subsequently separated with different types of electrostatic separators. However, size classification is such a task that people want to avoid unless it is necessary. Size classification by means of electrostatic techniques has been reported in literature. These techniques mainly deal with classifying dry, fine powder when conventional size classifying processes fail to provide satisfactory separation. For example, a prior art attempt to separate fine, dust-like particulate material is disclosed in U.S. Pat. No. 3,222,275 to Breakiron et al. According to this patent, very fine particulate materials that are of a mesh size of xe2x88x92200 are amenable to high-tension separation with a spray of mobile ions produced by a corona discharge.
Most techniques for classifying particulate materials employ the phenomenon that particulate materials become charged by means of induction when they are subject to a strong electric field. Size separation may thereby be achieved by passing charged particulate materials through electrified sieves. For example, U.S. Pat. No. 5,484,061 to Dunn discloses such an electrostatic sieving apparatus for classifying particulate materials according to size. U.S. Pat. No. 5,161,696 to Seider discloses an apparatus for separating shapes of abrasive grains by imposing a high-voltage corona induction charge to free-falling abrasive particulate materials.
In addition to particulate material size, operating parameters affect an electrostatic separator""s performance. Such operating parameters are roll speed, number of corona electrodes and their corresponding position with respect to the grounded electrode, intensity and polarity of applied potential, particulate material rate, electrode surface cleaning, temperature of the particulate materials, and splitter positions.
In view of the foregoing background, it is therefore an object of the invention to provide a high-tension electrostatic classifier and separator that may include a corona classification section for classifying feed particulate materials into a fine to middle size fraction and middle to coarse size fraction before such fractions are separated by a roll electrode separator and plate electrode separator, respectively. These and other objects, features, and advantages of the invention, are provided in a high-tension electrostatic separator for classifying and separating particulate materials based upon size and conductivity that may include a corona classifier that may have an elongated passageway having generally planar sidewalls defining a first end for receiving particulate materials and a second end for directing the particulate materials into two fractions according to size. The corona classifier may further include corona means located adjacent one of the sidewalls for providing ion bombardment in a horizontal direction to particulate materials dropping down the passageway so that middle to coarse size particulate materials travel in a more generally vertical direction and fine to middle size particulate materials travel in a less generally vertical direction, while passing through the passageway.
A splitter may be located in the passageway downstream of the corona means to direct middle to coarse size particulate materials in a first path toward the one sidewall and fine to middle size particulate materials in a second path toward another of the sidewalls. The splitter may be adjustable on an axis extending generally parallel to the sidewalls and perpendicular to a longitudinal axis of the passageway. Further, the separator may include means for receiving fine to middle size particulate materials and middle to coarse size particulate materials for separating the particulate materials into a plurality of distinct fractions.
The corona means may include a plurality of spacers extending from the one sidewall in a generally horizontal direction and between opposed sidewalls of the passageway. The sidewalls of the passageway may be conductive. A plurality of spaced corona electrodes extend adjacent and along the one sidewall and may have opposite ends connected to the plurality of spacers so that the plurality of corona electrodes are spaced from the one sidewall. The plurality of spacers are non-conductive for isolating the plurality of corona electrodes from the one sidewall.
A reservoir is located above the passageway for feeding particulate materials therein by gravity into a thin stream generally equal in width along and spaced from the one sidewall of the passageway. The corona classifier may further include a screen located within the passageway and connected to the splitter for providing enhanced separation of middle to coarse size particulate materials from fine to middle size particulate materials. The screen has a mesh surface for passing fine to middle size particulate materials therethrough and for inhibiting middle to coarse size particulate materials from passing therethrough. The screen may be nonconductive.
The splitter may include an upper edge portion for supporting the screen. Further, the screen may extend generally between opposed sidewalls of the passageway. The splitter may have a rotatable base generally opposite to the upper edge portion for pivoting the splitter and screen toward and away from the one sidewall and for moving the splitter upwardly and downwardly. The corona classifier section may further include a plurality of baffles extending along the length of the passageway and spaced from each other in the general path of the middle to coarse size particulate materials. The plurality of baffles assist in retarding the fall of the middle to coarse size particulate materials.
The corona classifier may further comprise a housing having a plurality of elongated and generally vertical members with respective first ends that are attached and extend from corresponding corners of a base member. The housing has a plurality of elongated and generally horizontal members for connecting to corresponding second ends of the plurality of generally vertical members so that the housing may define a hollow space for generally supporting the corona classifier therein. The housing may be conductive.
The present invention also provides a method for classifying and collecting particulate materials according to size. The method includes passing particulate materials through a passageway in close proximity to a corona source for charging thereof. The method further includes classifying particulate materials traveling through the passageway according to size so that particulate materials are directed into diverging paths with a first path being for fine to middle size particulate materials and a second path being for middle to coarse size particulate materials. The separated fine to middle size and middle to coarse size fractions may then be collected or further processed.
To further aid in classifying the particulate materials, an adjustable splitter and a screen attached thereto may be installed in the passageway for providing enhanced classification of fine to middle size particulate materials from middle to coarse size particulate materials. A plurality of spaced containers are placed adjacent to a respective path of middle to coarse size conductive particulate materials and middle to coarse size nonconductive particulate materials for collecting thereof. Similarly, a plurality of spaced containers are placed adjacent to a respective path of fine to middle size conductive particulate materials and fine to middle size nonconductive particulate materials for collecting thereof. The plurality of spaced corona electrodes should be coated with a nonconducting polymer for inhibiting electric shock when touched and for preventing arcing.
In an alternate embodiment, a high-tension electrostatic separator for classifying and separating particulate materials based upon size and conductivity is disclosed. The separator includes a corona classifier section that classifies particulate materials according to size and directs same to first and second separators.
The first separator section receives fine to middle size particulate materials from the first path of the passageway and separates same according to conductivity. The first separator section includes an elongated cylindrical, grounded, conductive body having a rotative longitudinal axis and a substantially smooth outer drum surface for receiving fine to middle size particulate materials thereon, means for rotating the body about the longitudinal axis, and shaft means extending outwardly from opposite ends of the body along the longitudinal axis. The first separator section further includes a splitter located spacedly therefrom and generally in the second quadrant for separating fine to middle size conductive particulate materials from fine to middle size nonconductive particulate materials. The splitter should be adjustable on an axis extending parallel to the longitudinal axis of the body.
A support frame is disposed outwardly of the corona classifier section and the first separator section. The frame includes a pair of journals to support the shaft means for the rotating body. The first separator section includes an alternating current wiper located generally in a third quadrant for removing fine to middle size nonconductive particulate materials from the outer drum surface. The first separator section further includes a rotatable brush generally midway of the third and fourth quadrants for removing any remaining fine to middle size particulate materials from the outer drum surface. The first separator section may also include a baffle located spacedly therefrom and generally in the third quadrant for directing fine to middle size particulate materials into a corresponding container.
A corona means is supported by the frame located spacedly above the outer drum surface and angularly downstream from depositing fine to middle size particulate materials on the outer drum surface. A plurality of spaced, elongated static electrodes extend adjacent and along the outer drum surface of the body and may have opposite ends supported by spaced arcuate buses. The plurality of static electrodes are positioned at selected locations within first and second quadrants of the cylindrical body for providing a static electric field for attracting fine to middle size conductive particulate materials from the outer drum surface while fine to middle size nonconductive particulate materials remain pinned to the outer drum surface for subsequent removal as the body rotates. Each of the plurality of static electrodes may be coated with a nonconductive polymer for inhibiting electric shock when touched and for preventing arcing.
The present invention further includes a second separator section for receiving middle to coarse size particulate materials from the second path of the passageway and for separating same into conductive and nonconductive fractions. The second separator section includes a curved, declining, grounded and conductive plate and a plurality of spaced electrodes spacedly located adjacent and above the plate for producing an electric field to attract and lift middle to coarse size conductive particulate materials from the plate while permitting middle to coarse size nonconductive particulate materials to travel by gravity on the declining plate.
The second separator section includes a splitter located spacedly between the plate and the electrodes for separating middle to coarse size conductive particulate materials from middle to coarse size nonconductive particulate materials. The splitter is adjustable on an axis extending parallel to the longitudinal axis of the plate.
Advantageously, the present invention provides corona-aided particulate material classification, an enhanced static electric field, a cylindrical, conductive rotative outer drum surface for separating fine particulate materials and a plate electrode surface for separating coarse particulate material. The present invention may further include a plurality of containers generally below the outputs from the high-tension electrostatic separator for respectively receiving middle to coarse size conductive particulate materials and middle to coarse size nonconductive particulate materials from the second separator section, and fine to middle size conductive particulate materials and fine to middle size nonconductive particulate materials from the first separator section. The plurality of containers may be nonconductive. The housing may further include means for removably securing the high-tension electrostatic separator thereto and generally within the hollow space of the housing.
Advantageously, the high-tension electrostatic classifier and separator may split narrower-sized fractions of particulate materials into more fractions according to conductivity. The present invention also provides an enhanced static electrode arrangement providing enhanced attraction force for separating fine conductive particulate materials. The side-by-side first and second separator sections improve separation efficiency and throughput capacity.
The present invention also provides a method for classifying and separating particulate conductive and nonconductive materials. The method may include passing particulate materials through a passageway in close proximity to a corona source for charging thereof. Particulate materials traveling through the passageway are classified according to size so that the particulate materials are directed into diverging paths with a first path being for fine to middle size particulate materials and a second path being for middle to coarse size particulate materials.
Separation of fine to middle size particulate materials into conductive and nonconductive fractions by use of a rotating, cylindrical and grounded outer drum surface is disclosed herein. Fine to middle size particulate materials are moved past a corona charging location so that conductors of fine to middle size particulate materials are removed from the outer drum surface by a plurality of spaced static electrodes. As a result, the nonconductors of the fine to middle size particulate materials remain on the rotating outer drum surface until they drop off or are removed from the outer drum surface prior to a full rotation of the outer drum surface.
The method includes separating the middle to coarse size particulate materials into conductive fractions and nonconductive fractions with a curved, declining grounded plate so that conductive middle to coarse size particulate materials passing on the plate are lifted off therefrom due to an electrical field produced by a plurality of spaced static electrodes located above and along the plate and are separated from nonconductive middle to coarse size particulate materials remaining on the plate and falling therefrom. The method further includes collecting the separated conductive fine to middle size fraction from the nonconductive fine to middle size fraction, and collecting the separated conductive middle to coarse size fraction from the nonconductive middle to coarse size fraction. Other method steps are disclosed by the summary of the apparatus claims, infra.
Advantageously, the present invention provides a method for classifying and separating particulate materials that may maximize throughput capacity, minimize particle misplacement, and enhance the effectiveness of the static field intensity produced by the plurality of static electrodes. By incorporating the corona classifier section with the first separator section (roll electrode separator) and the second separator section (plate electrode separator), a wide range of particulate materials may be effective and efficiently separated with one pass through the present invention.